<template>
    <ul class="list">
        <li class="item" 
        v-for="item of letters" 
        :key="item"
        :ref="item"
        @touchstart.prevent="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        @click="handleLetterClick"
        >
            {{item}}
        </li>
    </ul>
</template>

<script>
export default {
    name:'CityAlphabet',
    props:{
        cities:Object
    },
    computed:{
        letters(){
            const letters =[]
            for(let i in this.cities){
                letters.push(i)
            }
            return letters
        }
    },
    data(){
        return{
            touchStatus:false,
            startY:0,
            timer:null
        }
    },
    updated(){
        this.startY=this.$refs['A'][0].offsetTop
    },
    methods:{
        handleLetterClick(e){//当你点击某个字母的时候，handleLetterClick方法会接受到叫e的事件对象
          this.$emit('change',e.target.innerText)//向外触发事件
        },
        handleTouchStart(){
            this.touchStatus=true
        },
        handleTouchMove(e){
            if(this.touchStatus){
                if(this.timer){//函数节流
                    clearTimeout(this.timer)
                }
                this.timer=setTimeout(()=>{
                    const touchY=e.touches[0].clientY-79
                    const index=Math.floor((touchY - this.startY)/20)
                    if(index >= 0 && index < this.letters.length)
                    {
                        this.$emit('change',this.letters[index])
                    }
                },16)//延迟16毫秒执行，假设在16毫秒之间又去滚动该操作，那么将会清楚上一次的操作，并重新执行本次操作
            }
        },  
        handleTouchEnd(){
            this.touchStatus=false
        }
    }
}
</script>

<style lang="stylus" scoped>
@import '~styles/varibles.styl'
.list
  display flex
  flex-direction column
  justify-content center
  position absolute
  top 1.58rem
  right 0
  bottom 0
  width .4rem
  .item
    line-height .4rem
    text-align center
    color $bgColor
</style>